OSMdroid - unable to load offline map tiles from custom location

I have looked around and can't find an answer to what I hope is quite a simple thing to achieve this question seemed similar but has no replies! So the problem is this; I have my offline map tiles on the android device here:

Error downloading MapTile: /15/16000/9960
java.lang.IllegalArgumentException: n <= 0: 0
at java.util.Random.nextInt(Random.java:175)
at org.osmdroid.tileprovider.tilesource.OnlineTileSourceBase.getBaseUrl(OnlineTileSourceBase.java:33)
at org.osmdroid.tileprovider.tilesource.XYTileSource.getTileURLString(XYTileSource.java:19)
at org.osmdroid.tileprovider.modules.MapTileDownloader$TileLoader.loadTile(MapTileDownloader.java:171)
at org.osmdroid.tileprovider.modules.MapTileModuleProviderBase$TileLoader.run(MapTileModuleProviderBase.java:291)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:841)

Presumably because the local tiles aren't found and the MapTileProviderBasic ultimately falls to the MapTileDownloader and it barfs because there are no online tiles.

osmdroid looks inside the directory uuid for a subdirectory called 'tiles', although the documentation kind of tells you this, it's not very explicit. I think you could probably use any location on the sdcard using this approach, I just decided to use the simplest solution.